The invention relates to an adsorption-desorption process for the recovery of hydrogen from a feed gas that, in addition to the hydrogen, contains trace pollutants which are adapted to be strongly adsorbed in concentrations generally below 1% by volume and contains components which are adapted for only light adsorption in concentrations above 1% by volume.
Hydrogen containing feed gases occur among others in procedures such as the coke production, the coal gasification and the oil distillation. They are used in increasing amounts for the recovery of hydrogen. For this purpose preferably adsorbents are used which adsorb normally all gases except the hydrogen from the hydrogen containing feed gas. The hydrogen itself which is not adsorbed leaves the adsorbent containing vessel, generally called the adsorber, at a comparatively high degree of purity. In these feed gases there are trace pollutants which are adapted for strong adsorption and comprise among others higher hydrocarbons, NO.sub.x (x indicating the different nitrogenous oxides), H.sub.2 S, COS, mercaptans, NH.sub.3 and steam. The components which are adapted for less strong or only faint adsorption may be constituted by gases such as N.sub.2, CO, CO.sub.2, CH.sub.4, C.sub.2 H.sub.6, etc.
The components suited for only light adsorption can usually be easily desorbed while components which are adapted to be strongly adsorbed are desorbed only with certain difficulties.
As adsorbents there may be mentioned among others activated carbon, zeolithes, silicagel, aluminum hydroxide gel and carbon molecular sieves.
The adsorption capacity of any adsorbent at some point is exhausted. That is, it is completely contaminated by the adsorbed materials. However, since a disposal of the completely contaminated adsorbent is usually too costly, the adsorbent is from time to time subjected to a purification process to remove the adsorbed materials. This is called the desorption procedure.
Desorption is most easily carried out by isothermic pressure reduction, that is pressure release and/or evacuation or by rinsing (reduction of partial pressure). The rinsing can also be used in addition to the general pressure reduction. These procedures, however, result in the desorption only of the gases which are lightly adsorbed. The case is different with the strongly adsorbed gases which, because of their low concentration in the feed gases involved, will be called trace pollutants. These trace pollutants can be desorbed only by a temperature increase and/or steam rinse to a certain extent. If they are subject to decay in the pore system of the adsorbent because of a catalytic effect or if they react chemically (cracking, polymerization), they must be removed by a chemical reaction with a partially gasi-frying medium for instance steam or CO.sub.2 at temperatures of usually above 970 K. This removal procedure for the trace pollutants in the following will be designated "regeneration" of the adsorbent as distinguished from the clear out desorption by pressure reduction and/or partial pressure reduction and temperature increase.
Presently, there are frequently used two-stage adsorption processes for the recovery of hydrogen from hydrogen containing feed gases. In a first adsorption stage, the preliminary purification stage, the gases which are adapted for strong adsorption are adsorbed, while in a second adsorption stage, the main purification stage, apart from hydrogen, all other gases which are adapted for light adsorption only are adsorbed. The hydrogen itself is discharged from the second adsorber at a more or less high purity, usually at a purity above 99% by volume.
Generally, the crude or feed gas flows continuously through the preliminary purification stage under constant operating conditions and in this course of the stage is gradually saturated with highly adsorbable gases.
In other words, the adsorbent is gradually contaminated. After a certain time when the adsorbent has reached the state of substantially complete contamination it is, depending on the kind of deposition in the pore walls, heated or rinsed with steam at temperatures between 370 K. and 570 K. or reactivated at temperatures above 970 K. with a partially gasifying medium such as steam or CO.sub.2. For the latter process the adsorbent is removed from the adsorber. Occassionally regeneration is completely dispensed with for cost reasons and the contaminated adsorbent is simply replaced by new material.
After leaving the preliminary purification stage the mixture enters a second adsorption stage in which the hydrogen is separated from the remaining gases which are of the type adapted for light adsorption only. This is accomplished by passing the pre-purified gas mixture, usually under elevated pressure, through the adsorbent so that all components except the hydrogen are adsorbed and the hydrogen is discharged at a more or less high purity as the so-called product gas.
The desorption is effected alternately with the adsorption by pressure reduction or reduction of partial pressure when the adsorption capacity of the adsorbent has being reached for any of the gas components to be separated. This is called the breakthrough of this gas component.
The cycle periods of adsorption and regeneration in the preliminary purification stage usually are several days up to weeks. In the main purification stage the cycle times between adsorption and desorption are on the other hand in the range of a few minutes up to at most 1 or 11/2 hours.
In view of the substantially more expensive and more laborious regeneration in the preliminary purification stage as compared with the principal purification stage, efforts have been made to obtain a longer total service time, that is time prior to necessary regeneration in the preliminary stage adsorber. An overly rapid contamination of the adsorbent in the preliminary purification stage actually can interfere with the economy of the entire adsorption procedure.
The service time depends generally on the composition of the crude or feed gas, the quality of the adsorbent and the pressure and temperature of the gases. However, even if all these factors are pesent in their optimum form the service times for the preliminary stage are still much too short.
The present invention, therefore, has the object of obtaining a longer service time for the adsorbent in the preliminary adsorption stage in which the trace pollutants which are adapted for strong adsorption are being adsorbed.